(UNDATED) - Measles have returned in the U.S. to levels not seen in 20 years.

The CDC says there have been 288 cases of measles in the U.S. less than halfway into 2014. That's the highest number of cases for an entire year since 2000, when measles was declared "eliminated" by health officials.

It's the largest number for the first five months of a year since an unusual outbreak in 1994, and doctors believe the reason for it is simple.

"We're seeing more and more people refusing vaccines, and this is the natural consequence," said Dr. Christopher Belcher, infectious disease specialist at Peyton Manning Children's Hospital.

"Elimination" by CDC standards means there is no continuous transmission of a disease for at least 12 months in a specific area of the country.

While some areas of the U.S. have not been touched by measles, there have been large numbers of cases in parts of California and New York, and the areas of the outbreak have something in common.

"Many epidemics seem to center around communities where there are a large number of unvaccinated people, where the disease gets a foothold and spreads from there," Belcher said.

Misinformation spread about the safety of vaccines, usually on the Internet, is likely behind the drop in vaccine levels.

"Vaccines are not perfect, but when we get to high enough vaccination rates in everyone, then the transmission doesn't continue."

There have been no reputable scientific studies that say the measles vaccine, or the MMR (measles, mumps, rubella), shot causes any health problems, though with any shot, a few people may experience some side effects.

We typically get two measles vaccines in our lives - once around our first birthday and another just before starting school at age four or five.

Belcher says if you aren't sure whether you had the second vaccine, there's nothing wrong with getting another one.

"If you've been vaccinated and you're immune, you're just going to kill off the live-virus vaccine because of your immunity, and it won't cause any problems. If you haven't been vaccinated, you're doing the right thing to help stop the spread of the disease."